Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO or SnO:F) coatings are widely used as window electrodes in opto-electronic devices. These transparent conductive oxides (TCOs) have been immensely successful in a variety of applications. Unfortunately, however, the use of ITO and FTO is becoming increasingly problematic for a number of reasons. Such problems include, for example, the fact that there is a limited amount of the element indium available on Earth, the instability of the TCOs in the presence of an acide or base, their susceptibility to ion diffusion from ion conducting layers, their limited transparency in the near infrared region (e.g., power-rich spectrum), high leakage current of FTO devices caused by FTO structure defects, etc. The brittle nature of ITO and its high deposition temperature can also limit its applications. In addition, surface asperities in SnO2:F may cause problematic arcing.
Thus, it will be appreciated that there is a need in the art for smooth and patternable electrode materials with good stability, high transparency, and excellent conductivity.
The search for novel electrode materials with good stability, high transparency, and excellent conductivity is ongoing. One aspect of this search involves identifying viable alternatives to such conventional TCOs. In this regard, the inventor of the instant invention has developed a viable transparent conductive coating (TCC) based on carbon, specifically graphene.
The term graphene generally refers to one or more atomic layers of graphite, e.g., with a single graphene layer or SGL being extendible up to n-layers of graphite (e.g., where n can be as high as about 10). Graphene's recent discovery and isolation (by cleaving crystalline graphite) at the University of Manchester comes at a time when the trend in electronics is to reduce the dimensions of the circuit elements to the nanometer scale. In this respect, graphene has unexpectedly led to a new world of unique opto-electronic properties, not encountered in standard electronic materials. This emerges from the linear dispersion relation (E vs. k), which gives rise to charge carriers in graphene having a zero rest mass and behaving like relativistic particles. The relativistic-like behavior delocalized electrons moving around carbon atoms results from their interaction with the periodic potential of graphene's honeycomb lattice gives rise to new quasi-particles that at low energies (E<1.2 eV) are accurately described by the (2+1)-dimensional Dirac equation with an effective speed of light νF≈c/300=106 ms−1. Therefore, the well established techniques of quantum electrodynamics (QED) (which deals with photons) can be brought to bear in the study of graphene—with the further advantageous aspect being that such effects are amplified in graphene by a factor of 300. For example, the universal coupling constant α is nearly 2 in graphene compared to 1/137 in vacuum. See K. S. Novoselov, “Electrical Field Effect in Atomically Thin Carbon Films,” Science, vol. 306, pp. 666-69 (2004), the contents of which are hereby incorporated herein.
Despite being only one-atom thick (at a minimum), graphene is chemically and thermally stable (although graphene may be surface-oxidized at 300 degrees C.), thereby allowing successfully fabricated graphene-based devices to withstand ambient conditions. High quality graphene sheets were first made by micro-mechanical cleavage of bulk graphite. The same technique is being fine-tuned to currently provide high-quality graphene crystallites up to 100 μm2 in size. This size is sufficient for most research purposes in micro-electronics. Consequently, most techniques developed so far, mainly at universities, have focused more on the microscopic sample, and device preparation and characterization rather than scaling up.
Unlike most of the current research trends, to realize the full potential of graphene as a possible TCC, large-area deposition of high quality material on substrates (e.g., glass or plastic substrates) is essential. To date, most large-scale graphene production processes rely on exfoliation of bulk graphite using wet-based chemicals and starts with highly ordered pyrolytic graphite (HOPG) and chemical exfoliation. As is known, HOPG is a highly ordered form of pyrolytic graphite with an angular spread of the c axes of less than 1 degree, and usually is produced by stress annealing at 3300 K. HOPG behaves much like a pure metal in that it is generally reflective and electrically conductive, although brittle and flaky. Graphene produced in this manner is filtered and then adhered to a surface. However, there are drawbacks with the exfoliation process. For example, exfoliated graphene tends to fold and become crumpled, exists as small strips and relies on a collage/stitch process for deposition, lacks inherent control on the number of graphene layers, etc. The material so produced is often contaminated by intercalates and, as such, has low grade electronic properties.
An in-depth analysis of the carbon phase diagram shows process window conditions suitable to produce not only graphite and diamond, but also other allotropic forms such as, for example, carbon nano-tubes (CNT). Catalytic deposition of nano-tubes is done from a gas phase at temperatures as high as 1000 degrees C. by a variety of groups.
In contrast with these conventional research areas and conventional techniques, certain example embodiments of this invention relate to a scalable technique to hetero-epitaxially grow mono-crystalline graphite (n as large as about 15) and convert it to high electronic grade (HEG) graphene (n<about 3). Certain example embodiments also relate to the use of HEG graphene in transparent (in terms of both visible and infrared spectra), conductive ultra-thin graphene films, e.g., as an alternative to the ubiquitously employed metal oxides window electrodes for a variety of applications (including, for example, solid-state solar cells). The growth technique of certain example embodiments is based on a catalytically driven hetero-epitaxial CVD process which takes place a temperature that is low enough to be glass-friendly. For example, thermodynamic as well as kinetics principles allow HEG graphene films to be crystallized from the gas phase on a seed catalyst layer at a temperature less than about 700 degrees C.
Certain example embodiments also use atomic hydrogen, which has been proven to be a potent radical for scavenging amorphous carbonaceous contamination on substrates and being able to do so at low process temperatures. It is also extremely good at removing oxides and other overlayers typically left by etching procedures.
Certain example embodiments of this invention relate to a method of making a graphene thin film. A back support substrate is provided. A catalyst thin film is disposed, directly or indirectly, on the back support substrate. A hydrocarbon inclusive gas is introduced proximate to the catalyst thin film. The back support substrate is heated to cause the hydrocarbon inclusive gas to at least partially separate the carbon in the hydrocarbon inclusive gas and promote graphene growth in and/or on the catalyst thin film. The back support substrate is rapidly cooled to promote crystallization of graphene, directly or indirectly, on an outermost surface of the catalyst thin film, in making the graphene thin film.
Certain example embodiments of this invention relate to a method of hetero-epitaxially growing a graphene thin film. A back support is provided. A metal catalyst thin film is disposed, directly or indirectly, on the back support substrate. A hydrocarbon inclusive gas is introduced proximate to the metal catalyst thin film at a pressure of 5-150 mTorr. The back support substrate is heated to a temperature greater than about 700 degrees C. to cause the hydrocarbon inclusive gas to at least partially separate the carbon in the hydrocarbon inclusive gas. Graphite is grown on the metal catalyst thin film. The metal catalyst thin film is etched to form graphane using hydrogen atoms. The graphane is further etched to form graphene using hydrogen atoms.
Certain example embodiments of this invention relate to a graphene thin film hetero-epitaxially grown, directly or indirectly, on a metal catalyst thin film having a substantially single-orientation large-grain crystal structure, wherein the graphene thin film is 1-10 atomic layers thick. The graphene thin film may a sheet resistance of 150 ohms/square or lower, and the transmission of the graphene thin film may be reduced by less than 3% per atomic layer substantially uniformly across the visible and infrared spectra.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.